1. Field of the Invention
This invention pertains to the art of methods for hydraulically expansion-swaging a tube in a tubesheet such as is found in nuclear steam generators.
2. Description of the Prior Art
I am unaware of prior art relevant to the method of this invention. U.S. Pat. Nos. 3,977,086 and 3,979,810 disclose apparatus and methods for accomplishing basic hydraulic expansion-swaging. However, it is not believed that following the teachings of these patents would yield as satisfactory results from the tube expansion process as is obtainable with the method according to my invention.
It is desirable in hydraulically expanding a tube in a tubesheet bore, such as that of a nuclear steam generator, that there be essentially zero gap between the outer wall of the tube and the inner wall of the bore of the tubesheet after the expansion is completed.
It has been found in tube expansion studies in which the expansion pressure is progressively increased to expand a tube of the type used in nuclear steam generators against a tubesheet bore that as the pressure is progressively increased the tube basically yields to the tubesheet by about 10,000 psi (69.times.10.sup.6 Pa), but without any residual clamping force between the tube and the tubesheet. However, with the pressure then continuing to be progressively increased to a range of about 31,500 psi to 33,500 psi (217.times.10.sup.6 to 231.times.10.sup.6 Pa), essentially a zero gap is provided between the tube and tubesheet with a residual clamping effect being produced. This residual clamping effect is basically a result of the tube, which is of one material, continuing to yield while the tubesheet material remains plastic. Even though the growth of the tube internal diameter by the end of the initial yielding may be very large compared to the subsequent yielding, such as 15 mils growth at the 10,000 psi value with a subsequent further yielding of perhaps only 2 mils with the increase in pressure to the higher values, obtaining the essentially zero gap is considered highly desirable in nuclear steam generators to reduce the possibility of crevice corrosion in any gap.
A problem with progressively increasing the pressure to the high values to obtain the essentially zero gap is that there is a tendency for the seals of the expansion mandrel to be extruded. While one approach to minimize this is to use a first and a second mandrel with a greater and a smaller radial gap between the mandrel and the tube, and with the first mandrel being used with a lower pressure and the second mandrel being used with a higher pressure, this approach is not considered as desirable as the approach which is the subject of this invention.